Solid-State Lighting on the Fast Track

Photonics SpectraApr 2008Progress in technology development, standards and testing procedures has resulted in LED lighting applications five years ahead of schedule.

Mark McClear, Cree Inc.

One thing true in the semiconductor industry is that, when a problem is well-defined, the solution will be at hand shortly. A few years ago, the LED sector of the semiconductor industry turned its attention to the problem known as solid-state lighting. At that time, the conventional wisdom was that solid-state lighting would be about 10 years away. However, unprecedented progress in the past 16 months has reduced that time estimate to five years, and these advances have created an entirely new category of LED devices known as “lighting class” (Table 1).

Table 1. Lighting-class LEDs have many more performance requirements than traditional devices.Sixteen months ago, the brightest white-light LED that could be purchased commercially was around 50 lm at 350 mA, but advances in LED chip architecture have enabled the birth of lighting-class LEDs and have doubled the prior light output. This has been a critical enabler for outdoor lighting, because large arrays of individual LEDs are required to yield light output that is equivalent to that of conventional high-intensity discharge lamps. For example, with the older technology, an array of more than 200 LEDs would have been required to generate the same amount of light as a 175-W metal halide lamp. Arrays of LEDs of this size present a complicated electrical, thermal and mechanical design challenge and pose many difficulties in the field in terms of size, weight and wind resistance. Today, that same array would contain fewer than 100 LEDs, and the size, form factor and cost have scaled downward at the same rate that the brightness has scaled upward.

Stylish, high-performance LED outdoor fixtures have begun to appear, such as at this gas station in Racine, Wis. Courtesy of BetaLED Lighting.Besides being smaller and less expensive, the brighter LEDs also are more energy-efficient. Lighting-class LEDs of nearly 100 lm/W now are commercially available, and lighting companies have responded by developing fixtures that exceed the energy efficiency of traditional light sources by more than 35 percent. One surprising by-product of making the LEDs more efficient is that they generate less heat than the previous generation of LEDs, which also simplifies design and reduces cost.

At the same time that the LEDs have been getting brighter and more efficient, advances in packaging them, along with advancements in industry standards, have provided the tools to achieve long LED lifetimes and to quantify reliable lifetime predictions. One nagging problem with traditional outdoor lighting in the past has been maintenance costs — in some instances it costs nearly as much to change a burned-out lamp as it does to buy and install the fixture initially. Now, however, LEDs with predicted lifetimes in excess of 50,000 h (11 years of 12-hour-per-day operation) are available and, taken together with the brightness and efficiency gains, have delivered real customer value through energy savings and maintenance avoidance.

Bringing the light indoors

Indoor lighting technically is more challenging for LEDs than outdoor lighting. Skin tones and interior design appear more natural in warm-white color temperatures (3000 K) with a high color rendering index, and the human eye is very sensitive even to subtle changes in color in these regions. Distances also are much closer indoors, which affords less space for color mixing and for blending of nonuniform light. To access the indoor lighting market, the LED industry had to deliver — in addition to the advances in brightness, efficiency and lifetime previously cited — uniform warm-white LEDs with a high color rendering index and a stable color point that does not change or shift over time.

While these technology developments were under way, the US Department of Energy (DoE) began its luminaire testing program called CALiPER (Commercially Available LED Product Evaluation and Reporting). The first two rounds of testing, using the older generation of LEDs, showed that indoor solid-state lighting fixtures were less than impressive relative to compact fluorescents and, in fact, were not much more efficient than incandescent fixtures. The third round, however, documents the recent progress of LED technology in relation to color rendering index, color point stability and other light quality indices, as well. Round three of the CALiPER testing showed for the first time that the performance of some indoor solid-state lighting fixtures was superior to that of compact fluorescent luminaires in light output, color quality and energy savings delivered.

Indoor adoption also has been delayed by the lack of applicable lighting standards to govern and judge the quality of light produced by an indoor luminaire. To respond to this need, new standards are in development by the Illuminating Engineering Society of North America and by the American National Standards Institute for lumen measurement and maintenance (LED lifetime) and chromaticity.

The DoE also recently announced the new solid-state lighting Energy Star criteria, which should serve as a catalyst for the adoption of indoor solid-state lighting. The timing for this confluence of LED technology advancement, standards development and governmental support via the Energy Star program could not be more auspicious given the simultaneous backlash against mercury-containing high-intensity discharge, linear fluorescent and compact fluorescent technologies. The market is looking for truly green alternatives, and LEDs now stand ready to serve this need.

Looking forward, we expect further advances in LED technology over the next several years. The DoE road map for LEDs recently was revised because the agency predicts a nearly 50 percent increase in lumen-per-watt efficacy over the next four years, while cost per lumen is dramatically reduced simultaneously. Taken together, these factors should ensure that solid-state lighting continues on the fast track for many years to come.

Meet the author

Mark McClear is the director of business development at Cree Inc. in Durham, N.C.; e-mail: mark_mcclear@cree.com.